1. Field of the Invention
The present invention relates to a support mechanism and a feeding unit including such a mechanism, and more specifically to a support mechanism, which moves a pickup for optically writing and reading information on and from a recording surface of a recording medium, along the recording surface thereof, while supporting the pickup, as well as a feeding unit including such a support mechanism.
2. Discussion of the Background
In recent years, optical discs including CDs (Compact Discs), DVDs (Digital Video Discs), and MDs (Mini Discs) have been put into practical use as information recording media onto or from which information is written and read utilizing an optical beam such as a laser beam.
A device, which is known as a pickup, is used to radiate an optical beam onto an information-recording surface of the optical disc and receive the light reflected from the surface. While the pickup is moved in parallel with the information-recording surface in the radial direction of the optical disc, the optical disc rotates. This rotation enables bits of information to optically be written or read onto or from information trucks formed spirally on the optical disc.
Referring to FIGS. 13 to 16, a conventional feeding unit for moving the pickup in the radial direction of the optical disc in parallel with the information-recording surface will now be described.
FIG. 13 is a plan view illustrating the conventional feeding unit, FIG. 14 is a cross-sectional view cut along the line XIV—XIV in FIG. 13 (or FIG. 15) and FIG. 15 is a bottom view illustrating the conventional feeding unit. FIG. 16 is an enlarged cross-sectional view of a supporting mechanism included in the conventional feeding unit.
As shown in FIGS. 13 to 15, the conventional feeding unit S′ includes a pickup casing 100 on which a pickup 100a is secured. The pickup 100a has a lens 101 and various drive parts. The lens 101 is used to radiate an optical beam B and receive light that is reflected on an optical disc DK. The drive parts drive optical parts, such as a semiconductor laser, and the lens 101, both of which produce the optical beam B cooperatively, in the trucking or focus direction of the optical disc DK.
The pickup casing 100 has two sides facing to each other. On one side thereof, two main support members 102 are mounted so as to protrude from the side. On the other side thereof, one sub support member 105 is mounted.
A through hole is formed through each of the two main support members 102 that allow a main shaft 103, which is secured by a not-shown chassis or others, to move through the through holes in the direction parallel with the direction of movement of the pickup casing 100. The main shaft 103 passing through the through holes permits the one side of the pickup casing 100 to be supported movably.
On the other hand, the sub support member 105 is formed so that it is able to pinch a sub shaft 106 secured by a not-shown chassis or other members in the direction parallel with the main shaft 103. The sub support member 105 receives the sub shaft 106. This receipt enables the other side of the pickup casing 100 to be supported movably by the sub shaft 106.
On the outer circumferential surface of the main shaft 103, a feeding screw 103a is made in the form of a helical to move the pickup casing 100 along the feeding screw. A rack gear 111 engaging with the feeding screw 103a is provided at the tip of a pressing spring 112 secured at the bottom of the pickup casing 100 with the help of a screw 113.
The pressing spring 112 presses the rack gear 111 in a pressing direction shown by an arrow in FIG. 16. This makes it possible that the rack gear 111 engages with the feeding screw 103a. 
In the conventional feeding unit S′, the main shaft 103 is driven to rotate by an electric motor not shown, with the result that the rack gear 111 engaging with the feeding screw 103a travels along the main shaft 103. Concurrently with this travel, the sub support member 105 travels along the sub shaft 106, which allows the pickup casing 100 itself to move in the radial direction of the optical disk DK together with the pickup 100a. 
When the pickup casing 100 moves, delayed movement of the sub support member 105 relative to the movement of the rack gear 111 occurs due to friction between the sub shaft 106 and the surface of the sub support member 105, which comes into contact with the sub shaft 106. In such a case, force having a function of turning clockwise or counterclockwise the pickup casing 100 in FIGS. 13 and 15 is applied to the pickup casing 100. The two main support members 102 are provided so as to be apart from each other, with the result that the pickup casing 100 is supported on the main shaft 103. In addition, a resilient force is applied to the region of the main shaft 103 between the two main support members 102 by means of the pressing spring 112. It is therefore possible to prevent the pickup casing 100 from turning clockwise or counterclockwise in FIGS. 13 and 15.
Further, in the conventional feeding unit S′, in order to ensure a smooth movement of the pickup casing 100 with backlash of the pickup casing 100 restricted to a minimum, it is required that the through holes of the main support members 102 (through which the main shaft 103 passes) be worked with precision in relation to the diameter or other factors of the main shaft 103.
To realize such a higher precision, as shown in FIGS. 13 to 16, the conventional feeding unit S′ adopts a metal bush 110 tightly inserted into the through hole of each main support member 102. Then, working of a sizing treatment is performed to raise the precision of a coaxial degree to the main shaft 103. Alternatively, the through holes are subjected to the finishing treatment of the through holes, which is conducted as the secondary finishing treatment. In addition, in the case that the main support members 102 are formed of resin, it is required to use a mold produced with precision. Even when any finishing treatment is used, it is required that the through holes be formed under severe quality control.
However, the conventional feeding unit S′ having the above-described structure provides higher costs for the parts, because it is required that the inner surface of each through hole be formed with a high precision. In addition, in cases where suspended dusts go into the through hole and remain in a gap between the main shaft 103 and the through hole (see FIG. 16), it may be difficult for the pickup casing 100 to move smoothly.